1. Field of the Invention
The present invention relates to an image pickup apparatus and method using white balance processing.
2. Related Background Art
In an image pickup process performed by an image pickup apparatus such as a digital camera, a digital video camera, etc., a signal output from an image pickup element through a primary color filter is digitized by A/D conversion, and divided into blocks as shown in FIG. 6A. However, each block is configured by a unit including chrominance signals R, G1, G2, and B one by one as shown in FIG. 6B. For each block, a color evaluation value is calculated in the following equation.
                              C          x                =                              R            -            B                                Y            i                                                            C          y                =                              R            +            B            -                          (                                                G                  1                                +                                  G                  2                                            )                                            Y            i                                                            Y          i                =                              R            +                          G              1                        +                          G              2                        +            B                    4                    
As shown in FIG. 7, a white axis can be obtained by plotting the color evaluation values Cx and Cy by photographing white color in advance under color temperature from high color temperature to low color temperature (for example, refer to Japanese Patent Application Laid-Open No. 2003-244723). Since there is variance of white color in an actual light source, flexibility referred to as a “white detection range” is provided to white color. In this case, when the white detection range a block plotted on the axes Cx and Cy is included in the white detection range, it is assumed that this block is white. The integrals SumR, SumG1, FumG2, and SumB color pixels included in the white detection range are calculated, and a white balance coefficient is calculated using the following equation where kWB_R, kWB_G1, kWB_G2, and kWB_B respectively indicate the white balance coefficients of the chrominance signals R, G1, G2, and B.kWB—R=1.0/SumR kWB—G1=1.0/SumG1 kWB—G2=1.0/SumG2 kWB—B=1.0/SumB 
However, in the method described above, when there is no white portion on the screen, the integral of the color in the white detection range is nearly zero. Therefore, a white balance coefficient cannot be correctly calculated. Furthermore, there is lower probability of a white portion in zooming as compared with wide photographing, so that it is difficult to designate the color temperature of a light source. For example, when a camera zooms in to photograph a target person, the rate of white portions decreases on the screen.
Additionally, when a camera zooms in on a target person to photograph that person, the following problems also occur. That is, at high color temperature, color evaluation values are distributed around the area A as shown in FIG. 7. Therefore, the scene can be defined as a high color temperature light source. However, when the flesh color is plotted on the axes Cx and Cy under the light source, it is distributed on the low color temperature side in the white detection range. Therefore, there is a smaller white color area on the screen, and the color evaluation values are distributed in the area B shown in FIG. 7 when human's skin is zoomed in. That is, the problem is that the flesh color looks pale in a photographed image due to the wrong recognition of the flesh color as the white color at a low color temperature. Therefore, an unexpected image can be presented to a photographer.